According to this article, that also used B46 to achieve that high speed (along with N41 and B66).
https://www.lightreading.com/5g/t-mobiles-5g-reaches-1gbit-s-and-thats-just-the-beginning/d/d-id/759803?

Sprint's coverage map has been updated. It's a little "wonky" right now though. Some Sprint coverage is not showing up until I zoom in. It also looks like it's showing more T-Mobile roaming coverage (namely in Montana).